1. Field of the Invention
The present invention relates to vacuum pumping systems with a multistage Roots or xe2x80x9cclawxe2x80x9d multilobe dry primary pump, in which systems the inlet of the primary pump receives the gases to be pumped and the outlet of the primary pump discharges the pumped gases to the atmosphere or to a system for recycling the pumped gases.
2. Description of the Prior Art
Diverse industries, for example the semiconductors industry, employ fabrication processes in a controlled low-pressure atmosphere in a vacuum enclosure connected to a vacuum pumping system.
To establish and maintain a vacuum in the vacuum enclosure, the vacuum pumping system must, initially, pump a relatively large flow of gas to create vacuum; the vacuum pumping system then extracts from the vacuum enclosure the residual gases or the treatment gases intentionally introduced into the vacuum enclosure during the various controlled atmosphere fabrication process steps. The flows of gas to be pumped by the vacuum pumping system are then lower.
A permanent concern, in the semiconductors industry in particular, is to maintain a high purity of the gases contained in the vacuum enclosure. To this end, it is necessary to avoid retrograde pollution from the vacuum pumping system. In particular, this rules out the use of vacuum pumping systems including liquid ring pumps. In modern techniques, vacuum pumping systems are based on Roots or claw dry pumps.
On the other hand, the treatment gases introduced intentionally into the vacuum enclosure are frequently costly gases, and it is advantageous to recycle these gases at the outlet from the vacuum pumping system, by means of a pumped gas recycling system, in order thereafter to reintroduce them in a controlled manner into the vacuum enclosure. It is then necessary to avoid contaminating these gases as they pass through the vacuum pumping system, and this is a second reason for using Roots or claw dry primary pumps, rather than traditional primary pumps with an oil seal.
Accordingly, in prior art vacuum pumping systems using Roots or claw dry primary pumps, the inlet of the primary pump receives the gases to be pumped, either directly from the vacuum enclosure, or indirectly via a secondary pump, which can be a turbomolecular pump. The primary pump discharges the pumped gases directly to the atmosphere or directly to a pumped gas recycling system.
Diverse industries have to pump and recycle pure low thermal conductivity gases, such as argon or xenon. This is the case in the semiconductors industry in particular, in which these gases are used in light sources emitting in the far ultraviolet spectrum in photolithographic equipment for fabricating new generation electronic circuits.
In this type of application, these very pure gases are used at a low pressure in the vacuum enclosure, and are evacuated by a pumping system using a Roots multistage dry primary pump or a claw multilobe dry primary pump.
In a multistage pump, the gas to be evacuated is aspirated by the first stage of the pump and then compressed in subsequent stages to a pressure slightly greater than atmospheric pressure at the outlet of the last stage and then rejected to the atmosphere or discharged to a pumped gases recycling system.
It has been found that prior art vacuum pumping systems using Roots multistage dry pumps or claw multilobe dry pumps have a serious drawback if pure low thermal conductivity gases, such as argon or xenon, are introduced into the vacuum enclosure during process steps. This is because the presence in the pumped gases of a high content of pure low thermal conductivity gas, such as argon or xenon, leads very quickly to binding and destruction of the dry primary pump.
The fast binding and destruction of the pump are due to binding of the last stage of the pump, stage which discharges the gases at a pressure close to atmospheric pressure.
The explanation for this is found in the following analysis: in a multistage dry pump, regardless of its technology, the gas is compressed in the successive stages of the pump, from the aspiration pressure at the inlet of the first stage to atmospheric pressure at the outlet of the final stage. In each compression stage the gas is heated and heats the adjacent pump parts. The compression is not regular, however, and the greatest compression occurs in the final stage. A compression greater than 5xc3x97104 Pa is generally obtained in the final stage. It is thus in the final stage that the gas is heated the most and therefore that most of the energy in the form of heat must be dissipated.
The structure of dry primary pumps includes a stator in which rotate two mechanically coupled rotors and offset laterally relative to each other. The rotors are supported by bearings, and are separated from the stator by the thin layer of gas in the mechanical clearances between the rotor and the stator or the pump body. A very small portion of the heat in a stage of the pump is dissipated by conduction to the pump body through the shaft of the rotor, and the greater portion of the heat is dissipated by conduction through the thin layer of gas between the rotor and the stator.
When pumping low thermal conductivity gas, the gas opposes the transfer of heat between the rotor and the stator. As a result of this, in the final stage of the multistage primary pump, the temperature of the rotor quickly increases to a very high temperature, a consequence of which is expansion of the rotor so that it comes into contact with the stator, leading to binding and destruction of the primary pump.
To prevent this phenomenon, one solution that has already been proposed entails injecting into the intermediate stages of the pump a high thermal conductivity gas such as nitrogen or helium. However, these additive gases are then mixed with the pure gas, and prevent simple recycling.
Another prior art solution entails intentionally increasing the functional clearances of the final stage to lower its compression ratio and thereby reduce the heat to be evacuated. However, the pump is then no longer able to achieve the required performance, and it is therefore necessary to distribute the loss of compression ratio over a large number of supplementary stages, which leads to a complex and bulky pump.
The problem addressed by the present invention is therefore that of designing a new vacuum pumping system structure that avoids destruction of the dry primary pump when pumping a low thermal conductivity gas, that uses prior art multistage dry primary pumps without modifying them, and that, where applicable, retains the same recycling technique, thus avoiding the need to develop a new pump.
To achieve the above and other objects, a vacuum pumping system in accordance with the invention includes a Roots or claw multistage dry primary pump which has an inlet adapted to receive gases to be pumped and an outlet adapted to discharge pumped gases to the atmosphere or to a pumped gases recycling system. In accordance with the invention, the vacuum pumping system includes an additional pump which has an inlet connected to the outlet of the primary pump and an outlet that discharges to the atmosphere or to the pumped gases recycling system. A preliminary evacuation pipe is connected in parallel with the additional pump, and includes a check valve adapted to pass gases coming from the primary pump. The additional pump is a dry pump that uses a technology other than the Roots or claw technology and is adapted to withstand without damage the temperature increase due to the final compression of the pumped gases.
In a first embodiment, the additional pump is a membrane pump.
In another embodiment, the additional pump is a piston pump.
The additional pump must be rated so that it is capable of pumping all of the flow of gas passing through the vacuum pumping system during the steps of pumping a vacuum at low pressure, for example to pump the flow of process gases during low-pressure fabrication process steps executed in a vacuum enclosure.
The additional pump can preferably be rated so as to be just capable of pumping said flow of gas when pumping a vacuum at low pressure. An additional pump that is small and inexpensive can therefore be used which is nevertheless sufficient to eliminate the problem of destruction of the dry primary pump.
The preliminary evacuation pipe must be rated to pass the high gas flow during preliminary evacuation steps of a vacuum enclosure.
The vacuum pumping system according to the invention can be connected to a vacuum enclosure containing, or into which are injected, low thermal conductivity gases.
The low thermal conductivity gases can include argon or xenon.
The pumped gases are advantageously discharged at the outlet of the vacuum pumping system into a pumped gases recycling system. The pumped gas recycling system extracts and recycles said low thermal conductivity gases to re-inject them in a controlled manner into the vacuum enclosure.